Yes, more or less

Meh, wibbly-wobbly timey-wimey stuff

@Vitruvius  
Even if Deutsch is right, I can’t imagine the effects would scale to macroscopic levels. So you might be able to make sub-atomic particles spin the wrong way, but you still won’t be able to kill Sarah Connor.

@Background Human  
I tend to fall into the Lloyd camp, but I don’t have the background to say that Deutsch is necessarily wrong. His idea doesn’t seem to be intuitive, but that’s quantum mechanics for you.

@Background Pony #00C8  
Parallel lines don’t intersect.
 
Also, mastery of the Shift key is recommended when discussing the mutability of spacetime.

@Vitruvius  
The really weird thing is that there are multiple ways to uphold the Novikov principle when you factor in the magical world of quantum physics.
 
The version proposed by MIT’s Seth Lloyd is closer to the classical understanding: the past is fixed, and any attempt to change it must necessarily fail. The only added wrinkle is in the way the universe routes around potential paradoxes. Since the improbable is by definition more likely than the impossible, any attempt to create a paradox can be undone by bad luck, no matter how bizarre. So if you try to kill your grandfather, the gun will jam, the shot will miss, or in the most extreme case, the bullet will phase harmlessly through Grandpa via coordinated quantum tunneling.
 
The version proposed by Oxford’s David Deutsch, on the other hand, abuses the nondeterministic nature of quantum physics to allow otherwise paradoxical actions. For example, if you have a 50% chance of killing your grandfather in the past, that means you have a 50% chance of being born in the future to go back and kill your grandfather. As long as the math adds up, everyone’s happy, even if it means someone’s past can be influenced by a future that never actually happens from their perspective. The exact way that works depends on your interpretation of quantum physics–and since Deutsch is a many-worlds guy, that brings us right back to alternate universes.

@Background Pony #00C8  
Actually, this (Novikov’s model) is the only form of time travel that makes any logical sense unless you want to invoke alternate universes. It does assume that causality can have a complex topology, but who cares.

@RIG  
That’s the kind of time traveling stories I love: those where changes in the past might not change our present, but create a new timeline/reality where those changes take place. Same with the future: We might be able to visit a timeline where no original travel was made. Whatever changes we do on that future, we won’t be able to see them in ours, because we’ll be altering a different timeline than ours.  
Of course, in the big scheme of things, it’s better to keep those changes as minimum or not so important to prevent possible aftermaths of altering the fabric of time and space.

Discord: Everything that happens now, is happening now.  
Sweetie Belle: What happened to then?  
Disocrd: We passed then.  
Sweetie Belle: When?  
Discord: Just now. We’re at now now.  
Sweetie Belle: Go back to then.  
Discord: When?  
Sweetie Belle: Now.  
Discord: Now?  
Sweetie Belle: Now.  
Discord: I can’t.  
Sweetie Belle: Why?  
Discord: We missed it.  
Sweetie Belle: When?  
Discord: Just now.  
Sweetie Belle: When will then be now?  
Discord: Soon.  
Sweetie Belle: How soon?

@Turkleson  
who is to say that the tiwlight she saw was, in fact, herself? what if it was a twilight from another world altogether?

@Turkleson  
dude they already had three beings who were basically the same ponies who traveled different paths.

@Background Pony #00C8  
The worlds we have seen appear more connected because of the travel from the mirrors. They said in Reflection, the more you travel to a world the more in common those worlds will have. To the point where the two suns apparently merge together.
 
Plus, we already know time travel doesn’t change the future because of the “It’s About Time” episode. Twilight goes back in time to talk to herself because it already happened, she didn’t change anything. That proves the fixed timelines AND the multiverse can coincide with the same canon.

@Turkleson  
then there would be vsatly larger differences between them than one ruling and one being corrupted. here are too close dconnection to the worlds we have seen for that to be the case.
 
also let us not forget how discord’s powers work. he twists the rules around and the things he twists often end up back in theri proper place. could very much be that these events retconned themselves into never happening.

Well look at it this way. Let’s say the forces that be created all this different dimensions. Some of them are different stories and some are just different versions of the same story. They share distinct similarities but the choices and actions that make them different have already been decided by destiny and not by the choices and actions of each other. These dimensions were never meant to be connected which is why traveling through the mirrors upsets the balance.

@Turkleson  
then they would have absolutely nothing in common what so ever if that was the case and there would be nothing to connect them. if they were worlds with differnt paths then they could be connected by those alterante decisions.

@Background Pony #00C8  
Because those alternate realities already exist. All the different dimensions came to be at the same time and their differences simply became more apparent as time went on. The only way those dimensions can effect each other is if the boundaries between them is undone, like in ‘Reflections’ and Equestria Girls to a lesser extent.

@FanOfMostEverything  
actually this has a bit of a problem to it all. if there can only be one past then how can there be multiple realities? if time cannot diverge then how would it create another reality?

@FanOfMostEverything  
at least they say the pst is immutable. that means the future is as yet decided.

@FanOfMostEverything  
then why would you be able to even go there at all? sighs. i do not like how they do the idea of tyimetravle in this. it makes no sense even by time travel standards and questions the existence of free will and all that crap….. which clearly is not something to question in a world where individuality is tatooed to your ass.

And you thought it’d be bigger on the inside!

@FanOfMostEverything  
that is boring.

Single, inflexible timeline. Whatever you plan on doing in the past already happened, so the past is immutable. Got it.

@RIG  
Say hi to Dragonball Z time travel.

@RIG  
A manufactured timeline split, yes.
 
Also, what the next WoW expansion is based off of.

how about you can change the past…but that creates a different timeline where the changes take place, while your own present remains the same?